1. Field of the Invention
The present invention relates to a liquid supply device which applies a pressure to liquid and supplies the liquid, and a printing apparatus including the liquid supply device.
2. Description of the Related Art
In an inkjet printing apparatus of off-carriage type, a liquid container is mounted at a position different from the position of a carriage. The size of such a liquid container can be easily increased. In addition, since the weight of the carriage including a printing head can be easily decreased, a scanning speed can be increased. The off-carriage type is used for large-size printing or business use, in which a consumption of liquid is large and a printing speed is high.
A tube supply style provides stable and continuous liquid supply because a liquid container and a printing head are connected to each other through a tube. In many cases, the tube supply style employs a pressure supply system which applies a pressure to liquid in a passage and supplies printing head with the liquid. In the pressure supply system, the arrangement of the printing head and the liquid container is less restricted. Hence, the layout of units in the apparatus can be more freely determined.
In particular, an indirect pressure supply system, in which a pressure is applied to the outside of a liquid bag containing liquid, only uses a single pressure pump regardless of the number of types of liquid in the printing apparatus. Since the liquid does not directly contact parts defining a pump, the material of the parts can be freely selected. A pressure unit may be a bellows pump, a diaphragm pump, a tube pump, etc.
A technique, which controls a pressure of air for liquid supply with the indirect pressure supply system, is described in document 1 (Japanese Patent Laid-Open No. 2002-52737). In document 1, control is provided such that a pressure of pressure air is detected, and a pressure of a cartridge is selected between a printing condition and a cleaning condition with a larger liquid consumption. In particular, the control is provided as follows. In the printing condition, the pressure pump is driven until the pressure of the cartridge becomes a first preset pressure which can deal with a maximum discharge amount of liquid, and then the pressure pump is stopped. In the cleaning condition, the pressure pump is driven until the pressure becomes a second preset pressure which can deal with an exhaust amount of liquid, and then the pressure pump is stopped. The second preset pressure is higher than the first preset pressure. Also, document 2 (Japanese Patent Laid-Open No. 11-188894) focuses on a change in liquid viscosity depending on an ambient temperature. In a configuration described in document 2, a driving time of a pump is changed in accordance with an ambient temperature, to obtain a pressure corresponding to a temperature change.
The size of a printing apparatus has been desired to be decreased. The size of the printing apparatus with the indirect pressure supply system is also desired to be decreased. The printing apparatus may be placed on or beside a desk in an office or another location closely relating to the home life of a user. Printing apparatuses for photo printing and business use consume a large amount of liquid. A liquid bag with a large capacity is necessary for such a printing apparatus, and hence driving sound of a pump for applying a pressure to the liquid is increased. In the indirect pressure supply system, an ambient pressure of the liquid bag is decreased because liquid in the liquid bag is consumed or because of a pressure spontaneously leaks. Therefore, it is necessary to keep the ambient pressure of the liquid bag to a predetermined pressure during printing. The pressure pump is driven when the ambient pressure of the liquid bag becomes lower than a predetermined pressure, and the pressure pump is stopped when the ambient pressure becomes higher than the predetermined pressure. This operation is repeated.
A motor of a pressure pump of the related art is driven at a constant driving speed which reliably satisfies supplement of liquid. Documents 1 and 2 do not describe a driving speed of a pressure pump. In the configuration of either document, a driving time of a pressure pump is adjusted to change a pressure. In such a driving method, a motor is driven at an excessively high rotating speed. When the printing apparatus is used near the user, the user may have a problem of noise because large motor sound may be repeatedly turned on and off.